A pressure-generating device of this kind is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 11-139118. In a pressure-generating device (air pressure regulator for tire) described in this publication, a pump unit is disposed on a member which rotates with an axle for rotatably driving an axle hub of a vehicle; the pump unit performs a reciprocating motion in the axial direction of the axle; and one end of a piston of the pump unit abuts an inclined plane of a cam member that is nonrotatable in relation to a wheel. The piston of the pump unit reciprocates in association with the rotation of the wheel.
According to the arrangement of the pressure-generating device described in the above publication, the axis of the piston of the pump unit is radially offset by a predetermined distance from the axis of the axle. Thus, the radius of the inclined plane of the cam member in contact with one end of the piston must be equal to or greater than the amount of the radial offset of the axis of the piston, thereby raising difficulty in reducing the size of the pressure-generating device and thus involving poor mountability in a vehicle. Furthermore, in order to achieve good wheel balance (balance of rotation), a counter weight (balance weight) for canceling the weight of the pump unit must be provided. This also raises difficulty in reducing the size of the pressure-generating device.